Home decorator COTTONS are washable. I use synthetic web straps (like you find on backpacks), and always include a pocket for small stuff like mints or gum. Decorator samples often are close to 15"x18", and I use those as-is.
Serge top of pocket and hem; attach to side of bag before assembly.
Seam and serge sides and bottom before hemming top edge.
Fold each bottom corner, matching side seam to bottom seam, and stitch across a triangle to make flat bottom on bag.
After hemming top, attach ends of 18" straps halfway between center and side seams. I seal ends with a candle flame to prevent fraying, then stitch around a square with zigzag stitches.
Do not exceed the weight of 1 gallon of milk and they'll last for years.